Current glass tempering machines employ what are known as oscillating roller furnaces in which glass is heated mainly by radiation. In the tempering process the temperature of the glass is increased above the softening point of glass in order to enable the glass to be tempered. Said temperature is between 610 and 625.degree. C. depending on the thickness of the glass. The glass is then cooled at desired speed typically using forced convection whereby air jets are blown at the glass from above and from below. This method enables high heat-transfer coefficients, necessary when thin glass is concerned in order to achieve a sufficient temperature difference between the surface and center of the glass. Examples of oscillating roller furnaces are disclosed in Fl patents 83,072 and 86,407.
A problem with roller furnaces is that heat transmission from massive rollers to glass is predominant particularly at the initial heating stage. The glass is supported by the rollers all the time, and particularly at the initial heating stage, with the temperature difference between the hot rollers and the glass being considerable, heat transmission from the hot rollers to the glass by conduction is abundant. This makes the edges of the glass bend upwards, the contact surface between the glass and the rollers becoming quite indefinite. In this case, uniform heating of glass is almost impossible as the contact point where the glass touches the roller becomes heated at the initial stage significantly faster than the rest of the glass surface. This easily causes curving, making uniform heating with normal heating times almost impossible. Further, the surface pressure at the point of contact of the glass touching the roller becomes high enough to subject the glass to optical faults, i.e. white marks and scratches, marring the surface of the glass.
Furthermore, the conditions in the furnace change during the heating period in tempering furnaces provided with rollers. The temperature of the glass changes relative to time and, particularly, heat transmission from the rollers diminishes as the temperature of the glass approaches the temperature of the rollers. On the other hand, the temperature of the rollers falls at the initial stage of the heating period when the thermal transfer to the glass is at its highest. This causes the problem of keeping the heating of the upper and lower parts of the furnace balanced during the entire heating period.
Fl patent 62,043 discloses a method and device for preventing the curving of glass sheets in a furnace provided with rollers in a horizontal tempering plant. In accordance with the cited publication, an air current is generated on the upper surface of a glass sheet in order to intensify the convection heat effect directed at the upper surface of the glass sheet. The blowing serves to compensate for the active heat transmission caused by the hot rollers to the lower surface of the glass sheet at the initial heating period, but said blowing does not completely equalize the differences in the heat currents affecting the upper and lower surfaces, as the lower surface of the glass gets a stronger heat current from above, particularly during initial heating, causing the above mentioned problems.
Fl patent 83,072 also discloses a method and a device for preventing the curving of glass sheets in a furnace provided with rollers in a horizontal tempering plant. It is known from said publication to direct an expanded convection heat effect at the upper surface of a glass sheet by blowing air at the furnace from blowpipes disposed in the vicinity of the upper surface of the glass sheet. Said publication further discloses that the furnace space below the glass sheet is cooled by leading the air to be blown into the furnace via heat exchange pipes provided underneath the glass sheet to blowpipes located on the upper side. Said arrangement is, however, quite complex and does not provide control of the equalization of heat in the tempering furnace during the entire heating period.